This functionality already exists.
The List-Unsubscribe header should be in all of the marketing emails we send out.
I just sent myself a test to ensure this is being added correctly. It looks to be. If you send yourself a marketing email from your own portal, and show the headers in Gmail you should see a line something like this:
List-Unsubscribe: mailto:1axc8p5842pps049nxdek8jz2e05v9mgwv4xhy-pnicholas=hubspot.com@bf01a.hubspotemail.net?subject=unsubscribe
I unsubscribed using this button and the app listened to the event unsubscribing my contact record. It is a mailto link.
If you have an instance where this is not working you can contact Support by phone at 1 888 482 7768 ext. 3 or through the Help tab in the bottom right while signed into your HubSpot portal.